Chapter 10 (PART TWO)

Cupula Functionality: The cupula is a structure within the inner ear that bends opposite to the motion of the head, thereby stimulating sensory receptors to help maintain balance and equilibrium.

The Eye: The eye is the organ that contains visual receptors providing the sense of vision.
- Accessory Organs of Sight:
- Lacrimal Apparatus: Involved in tear production and drainage.
- Eyelids: Serve protective functions and facilitate eye movement.
- Extrinsic Muscles: Assist in moving the eye.

Functions of the Eyelid:
- Protects the eye from foreign objects.
Layers of the Eyelid:
- 4 layers:
- Skin: The thinnest skin in the body.
- Muscle: Contains orbicularis oculi (closes eye) and levator palpebrae superioris (opens eye).
- Connective Tissue.
- Conjunctiva: A mucous membrane lining the inner surface of the eyelids and covering the anterior surface of the eyeball, excluding the cornea.

Lacrimal Apparatus Components:
- Lacrimal Gland: Produces tears that lubricate and cleanse the eye.
- Tiny Tubules: Release tears over the surface of the eye.
- Canaliculi: Two small ducts that drain tears into the lacrimal sac; tears then proceed into the nasolacrimal duct and into the nasal cavity.
- Tears Composition: Contain an antibacterial enzyme called lysozyme.
Extrinsic Eye Muscles:
- Attachment: These muscles attach to the sclera.
- Functionality: There are 6 extrinsic eye muscles that allow eye movement in all directions.

Innervation and Functionality Table:
- Orbicularis Oculi:
- Innervation: Facial nerve (VII)
- Function: Closes eye
- Levator Palpebrae Superioris:
- Innervation: Oculomotor nerve (III)
- Function: Opens eye
- Extrinsic Muscles of the Eyes:
- Superior Rectus: Innervated by the oculomotor nerve (III) - Rotates eye upward and toward the midline.
- Inferior Rectus: Innervated by the oculomotor nerve (III) - Rotates eye downward and toward midline.
- Medial Rectus: Innervated by the oculomotor nerve (III) - Rotates eye toward midline.
- Lateral Rectus: Innervated by the abducens nerve (VI) - Rotates eye away from midline.
- Superior Oblique: Innervated by the trochlear nerve (IV) - Rotates eye downward and away from midline.
- Inferior Oblique: Innervated by the oculomotor nerve (III) - Rotates eye upward and away from midline.

General Structure: The eye is a fluid-filled hollow sphere consisting of distinct layers:
- Outer (Fibrous) Layer: Includes cornea and sclera.
- Middle (Vascular) Layer: Comprises the choroid coat, ciliary body, and iris.
- Inner (Nervous) Layer: Contains the retina.
Fluid Spaces: Each layer of the eye contains fluids to help maintain its shape.

Components:
- Cornea: Transparent front portion, accounts for the anterior 1/6 of the outer layer, helps in focusing light rays.
- Sclera: The white part of the eye, making up the posterior 5/6; composed mostly of collagen and provides protection and attachment for muscles.
- Optic Nerve and Blood Vessels: Pierces the sclera at the posterior aspect of the eye.

Choroid Coat:
- Vascular, darkly pigmented, and nourishes other eye tissues, keeping the interior dark.
Ciliary Body:
- Forms a ring around the eye's front, contains ciliary processes and muscles that hold the lens in position and adjust its shape for focusing.
Accommodation: The lens adjusts shape to facilitate focusing on nearby or distant objects.
- Relaxation of Ciliary Muscle causes lens to flatten for distant focus.
- Contraction of Ciliary Muscle allows lens to thicken for close focus.

Structure:
- A thin, pigmented diaphragm of smooth muscle; the colored part of the eye.
Function: Adjusts the amount of light entering through the pupil;
- Dim Light: Radial muscles contract, pupil dilates.
- Bright Light: Circular muscles contract, pupil constricts.
Aqueous Humor: Secreted by the ciliary body, fills space between the cornea and lens (anterior cavity), nourishes cells, and maintains shape.

Retina Composition: Contains photoreceptors (visual receptor cells), consists of multiple layers, continuous with the optic nerve at the back of the eye, forms the inner lining almost to the ciliary body.
- Macula Lutea and Fovea Centralis: The central depression within the macula is the point of sharpest vision.
Optic Disc: Medial to fovea, where nerve fibers leave the eye to form the optic nerve; often referred to as the blind spot due to the absence of photoreceptors.
Posterior Cavity: Largest eye compartment, bounded by lens, ciliary body, and retina, filled with vitreous humor which supports internal structures and maintains eye shape.

Refraction: The bending of light rays as they enter the eye, crucial for focusing images on the retina, particularly on the fovea centralis.
Components Contributing to Refraction: Both cornea and lens primarily refract light, though the humors contribute to a lesser extent.
Types of Vision:
- Emmetropia: Normal vision where light focuses sharply on the retina.
- Myopia (Nearsightedness): Light focuses in front of the retina, causing a blurred image; corrected with a concave lens.
- Hyperopia (Farsightedness): Light focuses behind the retina without convergence; corrected with a convex lens.

General Information: Photoreceptors are modified neurons acting as visual receptor cells.
Types of Photoreceptors:
- Rods:
- Function: More sensitive to light, ideal for dim lighting; responsible for black and white vision.
- Detail: Produce less precise images due to axon branching onto fewer nerve fibers; there are significantly more rods than cones in the retina.
- Cones:
- Function: Provide sharp images in bright light and enable color vision.
- Detail: Densely packed cones located in the fovea centralis, with no rods in that area, contributing to high visual acuity.